Because it's relative to the original value.

Your logic: 100% means 1. A 100% increase is x * 1.

Correct logic: 100% means 1. A 100% increase is x + (x * 1)

Your 30% example is the same x + (x * 0.3)

200% is x + (x * 2) for a total of 3 times the original amount.

Same goes for 1000% being x + (x * 10) for a total of 11 times the original amount.